Although various kinds of control systems are available for the AC servo-motor of a synchronous motor type depending on end uses, a control system based on a square wave current is advantageous from the viewpoint of cost, in a field where comparatively less accuracy is required.
FIG. 5 shows one example of the AC servo-motor referred to above.
In FIG. 5, there are included a synchronous motor 1 provided with a permanent magnet rotor and three-phase drive windings, a rotor position sensor 2 connected to the synchronous motor 1 for detecting the rotor position, a rotary encoder 3 for detecting the rotating direction and speed of the synchronous motor 1, and a power inverter circuit 4 including power transistors Tr1 to Tr6 for switching current to be fed to the respective drive windings of the synchronous motor 1 and diodes D1 to D6 connected in parallel to the respective transistors Tr1 to Tr6. There are also provided a DC power source 5, and a control section 6 for controlling the energization of the transistors Tr1 to Tr6 of the power inverter circuit 4, constituted by blocks as described hereinbelow.
Specifically, there are provided a distributing section 61 which receives signals from the rotor position sensor 2, a PWM circuit 62 and a forward/reverse decision circuit 63, and which outputs a signal indicating which of the transistors of said power inverter circuit 4 should be turned on. The PWM circuit 62 produces pulse width-modulated (referred to as PWM hereinafter) signals corresponding to a speed adjusting signal to be described later, while the forward/reverse decision circuit 63 outputs signals indicating in which direction of the forward and reverse directions, the torque of the synchronous motor 1 should be produced by the speed adjusting instruction to be described later. There are also provided a base drive circuit 64 for controlling the base current of transistors Tr1, Tr2 and Tr3 connected to a plus side of the DC power source 5, and another base drive circuit 65 for similarly controlling the base current of transistors Tr4, Tr5 and Tr6 connected to a minus side of the DC power source 5, a current detecting circuit 7 for detecting the current flowing through the drive windings of the synchronous motor 1, and a speed amplifier 8 for amplifying the difference between the speed instruction and the output signal of the rotary encoder 3. It is to be noted here that the output of the speed amplifier 8 is compared with the output of the current detecting circuit 7, and is applied to the PWM circuit 62 and the forward/reverse decision circuit 63 as the speed adjusting instruction.
In FIG. 6 showing the functions of the distributing section 61, (a), (b) and (c) represent positional signals which differ in their electrical phase angle by 120.degree. and which are outputted from the rotor position sensor 2; (d) denotes an output of the forward/reverse decision circuit, with a "High" level showing the forward rotation and a "Low" level indicating the reverse rotation; (e) represents an output signal of the PWM circuit 62, and (f) to (k) show output signals of the distributing circuit 61 to be applied to the base drive circuits 64 and 65.
Subsequently, functioning of the AC servo-motor having the constructions as described above will be briefly explained.
Now, consideration will be given to a case where the actual rotational speed of the synchronous motor 1 is low with respect to the speed instruction.
The output of the speed amplifier 8 obtained through comparison and amplification of the speed instruction and the signal of the rotary encoder 3, becomes an output in a direction of acceleration, while the speed adjusting instruction obtained through comparison of the output of said speed amplifier 8 with the output of the current detecting circuit 7 and applied to the forward/reverse decision circuit 63 and the PWM circuit 62, becomes an acceleration signal. Accordingly, the forward/reverse decision circuit 63 outputs the forward rotation decision, and simultaneously, the PWM output of the PWM circuit 62 moves in a direction to increase its ON duty factor, and consequently, the current flowing into the drive windings of the synchronous motor 1 is increased for acceleration. Since this acceleration is effected until the speed instruction and the signal of the rotary encoder 3 coincide with each other (correctly, steady-state deviation remains), the speed of the synchronous motor 1 ultimately becomes the value of the speed instruction.
Meanwhile, in the case where the speed of the synchronous motor 1 is higher than the value of the speed instruction, the output of the forward/reverse decision circuit 63 becomes the reverse rotation decision and the synchronous motor 1 is decelerated as will be seen from the foregoing description.
FIGS. 7 and 8 show only the control section 6 as picked up from the conventional motor control arrangement of FIG. 5, and represent two systems conventionally adopted for subjecting the base drive signal to the PWM modulation. In FIGS. 7 and 8, the distributing section 61 is constituted by a distributing circuit 61a which receives the outputs of the rotor position sensor 2 and the forward/reverse decision circuit 63 for outputting the base signal corresponding thereto and is also constituted by the AND circuits 61b and 61c.
FIGS. 9(a)-9(b) and 10 show functions of the circuit of FIG. 7.
FIG. 9(a) shows a case where the transistors Tr1 and Tr6 are ON, with the other transistors turned OFF in the state of the forward rotation, and the current is flowing through the U phase.fwdarw.W phase of the drive windings of the synchronous motor 1, and FIG. 9(b) shows the function in the case where the transistors Tr1 and Tr6 are turned OFF from the state in FIG. 9(a). Upon application of the PWM modulation, the functions of FIG. 9(a).rarw..fwdarw.FIG. 9(b) are to be repeated. In FIG. 10 showing the functioning waveforms, (d) represents the output of the forward/reverse decision circuit 63; (e) shows the output of the PWM circuit 62, (f), (g) and (h) denote the outputs of the base drive circuit 64; (i), (j) and (k) represent the outputs of the base drive circuit 65, and (l) indicates the waveform of the current flowing through the U phase.fwdarw.W phase of the drive windings of the synchronous motor 1. FIG. 10 may be regarded as the portions I and II in FIG. 6 as picked out, with corresponding symbols being used therebetween.
In the above prior art system, since the transistors at the plus side and minus side of the DC power source are both turned OFF during the PWM modulation, ripples in the current tend to be increased, thus resulting in a drawback such as the generation of electro-magnetic noise. This fact invites a particularly serious problem in a motor in which low noise operation is required.
FIGS. 11(a)-11(b) and 12 show functions of another system, i.e., the circuit of FIG. 8. FIG. 11(a) shows the case where the current is flowing in U phase.fwdarw.W phase of the drive windings of the synchronous motor 1 in the state of the forward rotation, and FIG. 11(b) represents the function in the case where only the transistor Tr1 is turned OFF from the state of FIG. 11(a). Upon application of PWM modulation, the functions of FIG. 11(a).rarw..fwdarw.FIG. 11(b) are to be repeated.
In FIG. 12 showing the functioning waveforms, (d) represents the output of the forward/reverse decision circuit 63; (e) shows the output of the PWM circuit 62; (f), (g) and (h) denote the outputs of the base drive circuit 64; (i), (j) and (k) represent the outputs of the base drive circuit 65, and (l) indicates the waveform of the current flowing through the U phase.fwdarw.W phase of the drive windings of the synchronous motor 1. FIG. 12 may be regarded as the portions I and II in FIG. 6 as picked out, with corresponding symbols being used therebetween. In this system, since the PWM modulation is applied only to the transistors connected to the plus side of the DC power source 5 (referred to as UPPER transistors hereinafter), the ripples of current flowing through the synchronous motor 1 are reduced during the forward rotation, thus making it possible to realize a silent motor with less electro-magnetic noise. On the contrary, however, during the reverse rotation, there occurs such a phenomenon that, even when the transistors at the plus side of the DC power source 5 are turned OFF, the current represented by (induced voltage of the motor).div.(impedance of the coil) continues to flow through the transistors at the minus side (referred to as LOWER transistors hereinafter) thereof. Therefore, although this system may be utilized for the control of a small-sized motor with a small induced voltage and a comparatively large coil impedance, it can not be applied to a large-sized motor in which the current is undesirably dissipated irrespective of the PWM modulation.
The present invention has for its object to effect reduction of the electro-magnetic noises during the forward rotation, and suppression of current during the reverse rotation through elimination of disadvantages inherent in the conventional systems of this kind.